Laser processing methods for cutting or eroding material are known in many technological areas. Generally, a material to be processed is directly irradiated with laser light having a suitable wavelength. The energy from the laser light is absorbed by and coupled into the material in order to evaporate the material. The numerous applications of such processes can include, for example, cutting sheet steel, manufacturing microelectronic components, and structuring thin-film modules.
Laser processing methods have specific applications in structuring thin-film solar modules. For example, a photoactive semiconductor material can be applied across a surface and subsequently subdivided into a number of segments that are serially interconnected. Each of the individual layers of the thin-film structure, such as the front side electrode, the semiconductor layer, and the back side electrode, must be subdivided in order to structure the module surface. Considering the different absorption and dispersion properties of these superimposed layers and their boundary surfaces, lasers provide a suitable tool for such structuring.
However, in subdividing the module surface, the back side electrode is structured in the last step of the process. As a result, such laser processing methods are disadvantageous because the photoactive semiconductor layer lying directly under the back side electrode can be electrically damaged during the last processing step. Specifically, given solar modules composed of an amorphous semiconductor material, there is a risk of a phase conversion resulting from a highly electrically conductive connection between the front and back side electrodes. Further, an electrical short results when highly conductive regions in the semiconductor layer produce a connection between the front and back side electrodes. Further, other damage to the semiconductor layer, such as thermal stressing, also reduces the electrical power of the solar module.
In order to avoid a reduction in the electrical power of the solar module, a lift-off technique is typically used to structure the back side electrode in a thin-film solar module. Such a lift-off technique includes applying a thin line of paste to the active semiconductor layer before depositing the back side electrode on the semiconductor layer. The paste is then mechanically removed from the semiconductor layer after drying, and part of the back side electrode above the paste is also removed.
However, such a lift-off technique has a number of disadvantages. For example, a photo-inactive area having a width greater than 0.5 mm is produced, due to the width of the paste strip. Further, the adhesion between the back side electrode and the active semiconductor layer is locally diminished, due to the lift-off technique. In addition, the photoconduction of the active semiconductor layer can be deteriorated by chemical substances contained in the paste. Moreover, the structuring process of the back side electrode requires several different steps.